The conventional fire detector simply detects occurrence of a fire by means of a sensor which senses smoke concentration and/or temperature change and outputs a voltage, the switching circuit of the detector being turned ON when the voltage exceeds or falls below a fixed value. An element with self-holding function, such as a thyristor, is generally used as the switching circuit of a fire detector.
In testing the conventional fire detector, the operator has to introduce smoke into the fire sensor or to raise temperature thereof in order to learn whether the sensor is operating normally or whether the set voltage for operating the switching circuit is normal. An alternative method for testing the operation of the fire sensor is known, in which an equivalent voltage is applied to the sensor instead of actual introduction of smoke, actual temperature rise, etc. Even in this case, however, the operator must go to the place where the fire sensor is installed and the test itself is complicated. The need for going to such trouble and for such complicated testing is recognized as a defect of the conventional fire detector. Another defect is that the conventional fire detector operates by a comparison of the output voltage of the sensor with a fixed value and, therefore, the operation is merely binary, that is, ON or OFF, and degree or variation in smoke concentration and in temperature rise cannot be known. A method has also been known in which the output voltage of a sensor is sent out after conversion into a digital value, but in such a case the sensor unit must be provided independently of the digital converting unit so that the overall apparatus is complicated and expensive.
Accordingly, the object of this invention is to provide a fire detector which can be checkable for normal operation from a remote location and which can ascertain change and variation in parameters, such as smoke concentration and temperature, etc., in real time, thus eliminating the defects of the prior art fire detectors.